1. Field of the Invention
The present invention relates to a lithographic apparatus and a device manufacturing method.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, print heads, micro or nano-fluidic devices, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation sensitive material (e.g., resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a collection of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or antiparallel to this direction.
In maskless lithography it is known to project the patterned beam onto a target portion of the substrate using a projection system that comprises an array of lenses arranged such that each lens receives and focuses a respective portion of the patterned beam. Each lens of the array of lenses thus projects a respective spot of radiation onto the substrate, and the array of lenses collectively projects a radiation pattern on the substrate. Such systems are generally referred to as microlens array or MLA systems. In these systems, the patterned beam is typically projected onto array of lenses through a beam expander that comprises a series of optical components and is arranged to provide a substantially parallel radiation beam.
It will be appreciated that an inherent disadvantage of using a MLA of the is that a portion of the patterned beam incident on the MLA is lost, i.e., a portion does not reach the target surface of the substrate. This lost portion is the part of the beam which falls between the lenses of the MLA (i.e. the part that falls on a masking structure of the MLA). To reduce the amount of the beam cross section that is blocked (i.e., to maximize the proportion reaching the target substrate) openings or windows in the masking structure of the MLA may be made as large as possible. However, even with the MLA having larger windows, the proportion of the patterned beam cross section being lost (i.e., not reaching the substrate) can still be about 21.5%. With this result, a maximum fill ratio for a rectangular array of circular lenses is 78.5%. In practice, the maximum achievable fill ratio maybe lower than 70%.
Therefore, what is needed is a more efficient MLA.